Modulator



April 4, 1939. F, E, TERMAN 2,152,753

MODULATOR Filed March 9, 1956 A700/Manu) /fv/Par INI/ENTORT FREDERICK TERM/1N.

ATTORNEYS.

Patented Apr. 4, 1939 UNITED STATES PATENT oFFlcE Application March 9,

7 Claims.

My invention relates to modulators, and more particularly to thermionic modulator tubes which operate in conjunction with high frequency gen erators or amplifiers by varying radio-frequency losses at the modulation frequency.

Among the objects of my invention are: To provide a simple and easily controlled loss modulation system; to provide a simple modulation system which can be readily applied to existing code transmitters; and to provide an improved type of loss modulator in which the degree of modulation obtainable, the linearity of modulation and the overall eciency is greater than heretofore obtained in loss modulation systems.

My invention possesses numerous other objects and features of advantage, some of which, together with the foregoing, will be set forth in the following description of specific apparatus embodying and utilizing my novel method. It ,is therefore to be understood that my method is applicable to other apparatus, and that I do not limit myself, in any way, to the apparatus of the present application, as I may adopt various other apparatus embodiments, utilizing the method, within the scope of the appended claims.

Referring to the drawing:

Figure 1 is a circuit diagram reduced to lowest terms, showing the modulator of my invention applied to a power amplifier output.

Figure 2 is a circuit diagram where the method is applied to the modulation of an oscillator output.

Figure 3 is a circuit diagram showing how the modulator of my invention may be applied to a high frequency transmission line.

Figure 4 is a graph illustrating certain tube characteristics.

Referring directly to Figure 1 for a more detailed description of the Various applications of my invention, a power amplifier tube I, preferably of the thermionic type, is connected to a source, such as a master oscillator, through inu put leads 2 2, one of which, as is well known, leads to the cathode of the tube, the other leading to the grid 3. The anode 4 of the tube is energized through a radio-frequency choke. 5, and a constant-current choke 6 from a source 1. The choke 6 is proportioned so that the current which ows through it is substantially constant insofar as any tendency toward variation at the 1936, Serial N0. 67,866

cuit comprising inductance 9 and condenser I0, this tank circuit having coupledto the inductance IlV thereof an output coil II, the ends of which may be connected to any convenient -radiator. Y

A losser tube I2, of a convenient size and of the fundamental three-electrode type, is connected directly across thetank circuit lI-Ill,- with the losser tube anode I4 connected to one side thereof and the losser tube cathode I5 con- 10A nected to the other side.

The losser tube grid I6 is connected to the losser tube cathode through a transformer secondary I'I and a bias source I8. Modulation frequency is applied from input leads I9 to a prilo mary 2li, coupled to secondary I1.

In Figure 2 somewhat `the same arrangement is shown except that the losser tube I2, with its associated input circuits, is connected across the tank circuit of a conventionalized os- 20 cillator tube 2|, with its associated coupling circuit 22 arranged to cause the tube 2l to selfoscillate. Power is taken from the oscillator circuit 22 in the usual manner through coupled coil II. In this case the oscillator tube is ener- 25 gized through a constant-current choke 6, the oscillator tube 2l Abeing parallel fed, the oscillator circuit being isolated from the anode potential by blocking condenser 8.

It will be seen by inspection of Figures 1 and 2 30y that the losser tube shunts across the tank circuit a resistance which can be varied by the modulating voltage acting on the grid I6 of the losser tube I 2. This causes a radio-frequency current,which the plate circuit of the losser 35 tube draws, to vary with the modulating voltage. Y

The operation of my modulating system is as follows: The grid bias of the losser tube I2 is normally adjusted so that when no modulat- 40 ing voltage is applied a part of the radio-fre-I quency output appearing in the tank circuit is absorbed by the losser tube I2. 'Ihe tank cir cuit power not absorbed by the losser tube then represents .the carrier output. If one hundred 45l per cent modulation is to be provided for, onel half of the total tank circuit power must be abl sorbed in the losser tube, While the remaining half represents the carrier. If the maximum degree of modulation desired is less than one 50 hundred per cent, a larger proportion of the total power is available as the carrier.

Upon application of a modulating voltage to the grid of the losser, the resistance shunted across the tank circuit of the power ampliiier 55 2(0r oscillator) tube is alternately increased and decreased in accordance with the modulation.

Now in an ordinary power amplifier or power oscillator, the tendency is for the amplitude of the voltage developed across the tank circuit to remain constant as the load on the circuit varies. while the d.c. plate current tends to vary with the load. Thus, in an ordinary circuit the lower the resistance of the losser tube, the greater would be the d.'c. plate current; while the more the resistance, the smaller would be the d.-c. plate current. 'Ihe amplitude of oscillations would then tend to vary only slightly even with large changes in the resistance of the losser. It is for this reason that previous types of loss modulators have not been very satisfactory.

This diiculty is overcome in Figures 1 and 2 by the addition of the constant-current choke 6, which keeps the plate current from varying at the modulation frequency. The result is that as the losser resistance shunted' across the output tank circuit in Figures l and 2 varies, the voltage across thisvcircuit must alsovary, since the d.-c. plate current remains fixed- Thus, if the resistance across the tank circuit is lowered considerably, the instantaneous eiTect is to absorb 4energy from the tank* circuit and to produce thereby a slightVv reduction in the tank circuit voltage. In an" ordinary power am'plier or oscillator this slight decrease in voltage would cause sufcient increase in d.c' plate current-to supply the large additional losses. However, the presence of the constant current choke 6 in Figures 1 and 2 prevents the increase in plate' current, so that the amplitude of oscillations decreases further until the voltage across the tank circuit has dropped to the point where the energy in the tank circuit iss'm'all enough' to be sustained with the original d.c. plate current.

If the resistance whichJ the loss tube shunts across the tank circii'it'isincreased, the reverse situation exists. The fi'rsteiectv ofV such a reduction of losses is to increase the amplitude of oscillations slightly, and if it were not for the constant current choke the`d.e. plate current would then drop. However, since the plate current cannot drop, the tank circuit receives excess energy that increases the amplitude of oscillations until an equilibrium is establishedl between the output power and the d.c. input power at the constant current. l

It will be noted that the usel of the constant current choke increases greatly the degree of modulation obtained with a given variation in loss, and also increases considerabl'ythe' linearity of modulation.

One of the principal merits of the constantcurrent system of lossk modulation, asv` contrasted Y with previousloss modulators; is inthe improved overall eiiiciency which is obtained. It will be toprevent the current throughit from increasing asa result of the greater load placed on the tank circuit, has induced across it a voltage that subtracts from the plate supply voltage. Hence, during the positive half of the modulation cycle the plate voltag'eis large and the power input to the tube is likewise large, while during the negative half of the modulation cycle the p-late Voltage is 10W, and the power input to the tube is correspondingly small. The battery, however, supplies constant power at all times. The meaning of this is that the choke stores energy during the parts of the modulation cycle when the amplitude of the wave envelope is low, and then delivers. this energy to the tube during the parts of the modulation cycle when the amplitude of the wave envelope is great. This action of the choke helps out the losser tube and reduces the work which it must do.

A numerical example will make the power relations somewhat clearer. It will be assumed that the plate efficiency of the power tube is constant throughout the modulation cycle. This is not strictly true but will be used inv the example because it simplifies the explanation greatly. It will be assumed that a carrier power of 300 watts is" desired, that the degree of modulation. to be obtained is seventy-live per cent, and that the plate supply voltage is 2,000 volts. When no modulation is applied the losser tube must absorb an amount of power equal-to the degree of modulation times the' carrier power, which is 300 0.75=225 watts. The power tube is, therefore, called upon to generate 300-1-225=525 watts of-radio frequencyY power at a plate supply potential of 2,000 volts. power tube. When" modulation is applied, the total radio-frequency power at the crest of the modulation cycle is, where` m=the modulation expressed as adecimal 300 1+m 2=918 watts for seventy-live per cent modulation. As the losser will be adjusted so that it absorbs little or no power at the crest of the modulation cycle, the 918 watts represents the total peak power generated by the power amplier or oscillator tube. It will be observed that of the 618 watts difference between the peak power and carrier power, 225 watts were released by reducing the losser absorption to zero while the remaining 393 watts are supplied from the constant-current choke. At the trough' of the modulation cycle the total output power required is, where mzthe modulation expressed as a' decimal 300v watts for seventy-five per cent modulation. The amplitude of oscillations is now one-fourth Aof the carrier amplitude, and the total radio frequency power generated is also one-fourth of the total power with no modulation, or

watts. lI'his is because at one-fourth amplitude the plate voltage has dropped to one-fourth normal, but the plate current is held constant by choke 6. The difference of 112.5 watts between this 131.25 watts and the required output of 18.75 watts, represents the energy which the losser tube must absorb.

This determines the size of f CFI These results show that in order to obtain 300 .v

al plate-modulated power amplifier (or oscilla- 7'5'7g CII tor) using a class A modulator system, the total dissipation would be approximately 600 watts for power tube and modulator taken together, while ii class B modulation were employed the total plate dissipation which must be provided is of the order of 270 watts. From the point of overall plate efficiency it is, therefore, apparent that the loss system of'modulation herein described stands between the class A and class B modulation systems.

It is to be noted that the losses in the losser tube are maximum when' the wave is unmodulated, and decrease as theV modulation is applied. This results from the fact that the source of plate voltage supplies the same current and hence the same power irrespective of the presence or absence of modulation, but. since the modulated wave contains more energythan the unmodulated wave, the losser tube obviously absorbs less energy during modulation. This result is clearly brought out from the example considered above, as it will be noted that the losser tube absorbes no energy at the peak of modulation and absorbs only half as much energy at the trough of modulation as when no modulation is present.

One of the distinguishing features of the constant-current loss modulator is the ease with which it can be applied to a transmitter. The losser tube can be connected anywhere in the circuit, as for example across the radio-frequency transmission line, across the tank circuit, or across a tuned circuit coupled to the transmission line (see Fig. 3). In order to apply the modulation system to a code transmitter it is merely necessary to add the constant-current choke to the plate-supply system of the power tube, and to couple in the losser at some accessible point. In particular, it will be noted that the losser may even be an external unit separate from the transmitter mounting.

In Figure 5 I have shown the losser tube inserted across a transmission line25, this transmission line being fed from any convenient circuit 26 carrying radio-frequency power. In this case I prefer to couple the anode circuit of the losser tube l2 to the transmission line through a transformer 21. In this figure I have not shown the insertion of the constant-current choke in the plate supply system of the power tube, inasmuch as this power tube and circuit therefor may be any one of a large number of kinds.

For best results the losser tube should be operated in such a way that a high radio-frequency voltage is applied to its plate. The losser tube 1 should also have characteristics such that the losses increase very rapidly when the instantaneous plate voltage is greater than the plate voltage at which current begins to ow to the losser plate. Such a characteristic is indicated by the solid lines 30 in Figure 4, whereas the dotted lines 3l represent an undesirable characteristie. It will be noted that what is desired is essentially a high mutual conductance. Tubes with low amplification factor are therefore in general to be preferred.

The bias on the losser tube is so adjusted in operation that when no modulation is applied the losses have the proper value to provide for the degree of modulation' desired. With this adjustment, together with a tube having a steep characteristic such as shown by the solid lines of Figure 4, any variation in grid potential produced by the modulating voltage will tend to cause the amplitude of oscillations to vary almost exactly with the variation in grid bias. This is because discharge device constant.

the plate potential'at which plate current starts to flow in the losser is proportional to the grid bias, and if the losses in the tube increasevery rapidly as the plate potential exceeds the cutoi value, then the amplitude of oscillations will v always be such as to drive the plate of the losser tube just slightly beyond the point at which plate current starts to ilow. Such a condition assures that the modulation will follow the modulating voltage and hence be substantially distortionless.

It will be noted that the essential feature of the modulator herein described is the operation with constant d.c. plate current, and that the desirable results which Iare obtained are a consequence of the cooperation of the constant-current choke and the losser system. It is this feature that distinguishes my new type modulator from previous loss modulators.

I claim:

1. In combination with an electron discharge device having a circuit to which oscillating power is supplied by said device, means for modulating said power which comprises an absorber modulator connected across at least a portion of said circuit to which oscillating power is supplied, means for supplying a modulating frequency to said absorber modulator, and means for maintaining the direct anode current of said electron 2. In combination with an electron discharge device having a circuit to which oscillating power is supplied by said device, means for modulating said power which comprises an absorber modulator tube having a cathode, anode, and control electrode, said anode and cathode being connected across at least a portion of said circuit to which oscillating power is supplied, means for supplying a modulating frequency to the control electrode of said absorber modulator tube, and a choke in the direct current anode supply circuit of said electron discharge device to maintain a constant current therein.

3. In combination with an oscillator having an electron discharge dev-ice and a circuit to which oscillating power is supplied by said device, means for modulating said power which comprises an absorber modulator tube having a cathode, anode, and control electrode, said anode and cathode being connected across at least a portion of said circuit to which oscillating power is supplied, means for supplying modulating frequency to the control electrode of said absorber modulator tube, means for altering the magnitude of saidv oscillating power in accordance with the charge on said absorber modulator tube control electrod-e, and means for maintaining the direct anode current constant on the electron discharge device of said oscillator.

4. The method of modulating the high frequency output of an electron discharge oscillator which comprises maintaining a constant direct anode current to said oscillator, absorbing a variable amount of power from said high frequency output only, and reducing the instantaneous potential across said absorbing means in proportion to the modulation energy.

5. In combination with an electron discharge device and a circuit in which oscillating power is controlled by said electron discharge device, means for modulating said power which comprises an absorber modulator tube and circuit connected to said oscillating power circuit for enabling absorption of oscillating power by said modulating means, means for supplying modulating frequency to said absorber modulator tube for controlling Such labsorption 'of oscillating power, and means for maintaining the direct anode current to said electron discharge device substantially constant.

6. In combination with an electron discharge device and a circuit in which oscillating pow-er is controlled by ksaid electron discharge device, means for modulating said power which comprises an absorber modulator connected to said oscillating power circuit at points diering in instantaneous potential for enabling absorption of oscillating power by said absorber modulator, means for supplying modulating frequency to said absorber modulator for controlling such absorption of oscillating power, and a choke in the direct current anode supply circuit of said power electron discharge device to maintain a rconstant anode current therein.

7. In combination with a power tube having a circuit to which oscillating :power is supplied by lsaid tube, means for modulating 'said power f yill) 

